Heretofore, as a writing ink or jet printing ink used for recording on materials to be recorded (e.g. paper, etc.) or an ink for dyeing fibers, hides and the like, various polyazo dyes have been employed.
The polyazo dye consists mainly of direct dyes and, therefore, a dyeing process is simple and fastness is comparatively excellent. Polyazo dyes having such advantages are widely employed in the industry.
However, regarding the polyazo dye, there are problems to be solved. Firstly, there is a problem as to chemical properties, because excellent chemical properties such as stable solubility of dye, stability of concentrated dye solution with time and stability of dye density with time are required for dyes which are used for writing ink.
Secondly, there is a problem about safety, because safety of chemical substances has recently been noted and dyes having excellent safety which have no bad influence upon the human body are required.
As the dye wherein dyeing properties and fastness are improved, for example, benzidine or benzidine derivative based black trisazo dyes or polyazo dyes (e.g. C.I. Direct Black 19, C.I. Direct Black 154, C.I. Direct Black 168, etc.) have been widely used. However, these dyes have problems as to safety for humans because the Ames test gives false positive or positive results.
On the other hand, as useful polyazo black dyes with improved safety for humans are, for example, food colors (e.g. recording solution wherein Food Black 2 is used as a recording agent, etc.), 4,4'-diaminodiphenylamine-2-sulfonic acid (substituent for benzidine) based trisazo dyes, stilbene tetrakisazo dyes having no amino group in a molecular structure and the like. However, an aqueous ink wherein these dyes are used has poor water and light resistance and, therefore, dyeing properties and fastness are inferior.
Polyazo dyes which satisfy the above properties have never been obtained.
Further, water-soluble dyes may be separated from aqueous systems by means such as aciding-out, salting out and the like. However, it is difficult to obtain dyes having high purity because of inclusion of inorganic salts upon separation, whereby, various problems are caused. For example, when a dye having low purity is used for an aqueous writing ink, there is a disadvantage in that the ink is not consistently fed to a pen point. Also, when a dye having low purity is used for a jet printing ink, clogging of the outlet orifice may occur.